It is conventional practice in the recovery of crude oil, following possible degasification of oil extracted from the ground, to initially store the oil without further treatment in storage tanks, and keep it there ready for distribution. The tanks may hold 100,000 m.sup.3 fluid. The oil may be left sufficiently long in the tanks for considerable sediment to form, particularly under extreme climatic conditions. Crude oil as a natural product may have widely varying composition. The frequency of this sedimentation, the formation and nature of the sediments differ widely. In case of a conventional circular cylindrical crude oil tank with a diameter of approximately 100 meters, typical in the oil extraction industry, sedimentation layers of even a few dozen centimeters represent a loss of crude oil and, further, a substantial disposal problem. Sediment layers with a thickness of 1 to 1.5 meters, for example, are not infrequently encountered, particularly if, after various removal cycles, crude oil is constantly introduced without completely emptying the tank or without considering possible prior sedimentation.
The nature of the sedimentation depends on the type of crude oil. The sediment can be constituted by deposited asphalts or paraffins, waxes or other highly molecular weight hydrocarbons. The sediment may, however, also consist soley of thickened crude oil fractions. Crude oil thickens under various influences, for example under heat. This is a specific problem in desert areas. The oil sludge which forms may be of yogurt-like consistency. It can be considered as a crude oil fraction and largely consists of crude oil or thickened fractions which are re-dissolvable into crude oil.
This oil in sludge form is an undesired material. It reduces the tank capacity and clogs pumps. The material must be removed from the tank which, for example, involves cleaning the tank after it has been pumped empty.
It has been proposed to force compressed oil into the sludge to disintegrate the sludge by the force of the spurted compressed oil. U.S. Pat. No. 3,436,263, Strenkert et al, describes a process in which a cleaning material with which the oil residues are dissolved or removed in combination therewith is introduced. The final disposal of the sediment generally involves placing the oil sludge in a tank which is sacrificed for this purpose. Reprocessing of the oil sludge is not systematically considered or carried out.
French Patent 2,211,546 discusses dissolving sediments with foreign chemical substances. This may become a problem for the refinery operator since oil refineries are generally set up specifically for the treatment of crude oil. The equipment for such treatment operates with parameters adjusted in accordance with the source of the product to be processed. Foreign substances, such as dissolving chemicals which are introduced, may impede refinery operations. Refinery operators usually refuse to accept oil which is contaminated with such solvents.
Sludge which has been removed by chemical dissolution thus must be disposed, which is undesirable from an environmental standpoint; otherwise, a constant reduction of total storage capacity of the tank must be accepted.